Physical foundations of the method of atomic emission spectroscopy. Methods of atomic spectroscopy

The practical goal of atomic emission spectral analysis is quality, semiquantitative or quantitative determination of elemental composition analyzed sample. This method is based on the registration of the intensity of light emitted during the transitions of the electrons of an atom from one energy state to another.

One of the most remarkable properties of atomic spectra is their discreteness (linear structure) and the purely individual nature of the number and distribution of lines in the spectrum, which makes such spectra an identification feature of a given chemical element. Qualitative analysis is based on this property of the spectra. In quantitative analysis, the determination of the concentration of an element of interest is carried out by the intensity of individual spectral lines, called analytical.

To obtain an emission spectrum, the electrons that make up the particles of the analyzed substance must be given additional energy. For this purpose, a spectrum excitation source is used, in which the substance is heated and evaporated, the molecules in the gas phase dissociate into neutral atoms, ions and electrons, i.e. the substance is transferred to the state of plasma. When electrons collide with atoms and ions in a plasma, the latter pass into an excited state. The lifetime of particles in an excited state does not exceed 10 -10 s s. Spontaneously returning to a normal or intermediate state, they emit light quanta that carry away excess energy.

The number of atoms in an excited state at a fixed temperature is proportional to the number of atoms of the element being determined. Therefore, the intensity of the spectral line I will be proportional to the concentration of the element being determined With in sample:

where k- proportionality coefficient, the value of which depends non-linearly on temperature, ionization energy of an atom, and a number of other factors that are usually difficult to control during analysis.

In order to eliminate to some extent the influence of these factors on the results of analysis, it is customary in atomic emission spectral analysis to measure the intensity of an analytical line relative to the intensity of a certain comparison lines (internal standard method). An internal standard is a component whose content is the same in all standard samples, as well as in the analyzed sample. Most often, the main component is used as an internal standard, the content of which can be approximately considered equal to 100% (for example, when analyzing steels, iron can serve as an internal standard).

Sometimes a component that plays the role of an internal standard is deliberately introduced in equal amounts to all samples. As a comparison line, such a line in the spectrum of the internal standard is chosen, the excitation conditions of which (excitation energy, temperature effect) are as close as possible to the excitation conditions of the analytical line. This is achieved if the comparison line is as close as possible in wavelength to the analytical line (DA, homologous pair.

The expression for the relative intensity of the spectral lines of two elements can be written as

where index 1 refers to the analytical line; index 2 - to the line of comparison. Assuming the concentration of the С2 component, which plays the role of an internal standard, to be constant, we can assume that a is also a constant value and does not depend on the spectrum excitation conditions.

At a high concentration of atoms of the element being determined in the plasma, the absorption of light by unexcited atoms of the same element begins to play a significant role. Such a process is called self-absorption or reabsorption. This leads to a violation of the linear dependence of the line intensity on the concentration in the region of high concentrations. The influence of self-absorption on the intensity of the spectral line is taken into account empirical Lomakin equation

where b- the parameter characterizing the degree of self-absorption depends on the concentration and, as it increases, monotonically changes from 1 (no self-absorption) to 0. However, when working in a fairly narrow concentration range, the value b can be considered almost constant. In this case, the dependence of the spectral line intensity on the concentration in logarithmic coordinates is linear:

The Lomakin equation does not take into account the influence of matrix effects on the intensity of the spectral line. This effect is manifested in the fact that often the value of the analytical signal and, consequently, the result of the analysis depend not only on the concentration of the element being determined, but also on the content of associated components, as well as on the microstructure and phase composition of the analyzed materials.

The influence of matrix effects is usually minimized by using standard samples that are as close as possible in size, structure, and physicochemical properties to the substance under study. Sometimes, when analyzing trace impurities, matrix effects can be avoided by using the addition method and thorough homogenization of all samples.

Spectra excitation sources. The main sources of excitation of spectra in atomic emission spectroscopy include a flame, an arc of direct or alternating current, a spark, and an inductively coupled plasma.

The most important characteristic of the spectrum excitation source is its temperature. Temperature mainly determines the probability of particle transition to an excited state with subsequent light emission and, ultimately, the magnitude of the analytical signal and the metrological characteristics of the technique.

Flame . A variant of atomic emission spectroscopy using flame spectra as an excitation source is called the method flame photometry.

Structurally, the flame source of excitation is a gas burner, in which the analyzed sample (solution) is injected into the flame using a nozzle. The flame consists of two zones: internal (reduction) and external (oxidizing). In the reduction zone, primary reactions of thermal dissociation and incomplete combustion of the components of the combustible mixture occur. This zone contains many excited molecules and free radicals that intensely emit light in almost the entire optical range, from the UV to the IR region of the spectrum. This radiation is superimposed on the spectral lines of the analyte and interferes with its determination. Therefore, the reduction zone is not used for analytical purposes.

In the oxidizing zone, reactions of complete combustion of the components of the gas mixture take place. The main part of its radiation is in the IR range and therefore does not interfere with the determination of spectral lines in the UV and visible ranges. As a result, it is the oxidation zone that is used for analytical purposes. The temperature, composition and redox properties of the flame can be controlled within certain limits by changing the nature and ratio of the combustible gas and the oxidizer in the mixture. This technique is often used to select the optimal conditions for excitation of the spectrum.

Depending on the nature and composition of the combustible mixture, the flame temperature can vary in the range of 1500-3000°C. Such temperatures are optimal for the determination of only volatile and easily excitable elements, primarily alkali and alkaline earth metals. For them, the flame photometry method is one of the most sensitive (the detection limit is up to 10 wt.%). For other elements, the detection limits are several orders of magnitude higher.

An important advantage of the flame as a source of excitation of the spectrum is its high stability and the associated good reproducibility of the measurement results (the error does not exceed 5%).

Electric arc. In atomic emission spectroscopy, a direct or alternating current arc can be used as a spectrum excitation source. The arc source is a pair of vertically located electrodes (most often carbon), between which an arc is ignited. The lower electrode has a recess in which the sample is placed. In the analysis of metals or alloys, the lower electrode is usually made from the analyte. Thus, the arc discharge is most convenient for the analysis of solid samples. For the analysis of solutions, they are usually evaporated together with a suitable powdery collector, and the resulting precipitate is placed in the electrode recess.

The temperature of the arc discharge is significantly higher than the temperature of the flame (3000-7000°C), and for the AC arc the temperature is somewhat higher than for the DC arc. Therefore, atoms of most elements are effectively excited in the arc, with the exception of the most difficult to excite non-metals, such as halogens. In this regard, for most elements, the limits of detection in an arc discharge are one to two orders of magnitude lower than in a flame.

Arc sources of excitation (especially direct current), unlike flame sources, do not differ in high stability of the operating mode. Therefore, the reproducibility of the results is low (the error is 10-20%). However, for semi-quantitative determinations, this is quite sufficient. The best use of arc excitation sources is a qualitative analysis based on a survey spectrum.

electric spark. The spark source of excitation is arranged absolutely similarly to the arc source. The difference lies in the operating modes of the electronic circuit. Like the arc source, the spark excitation source is intended primarily for the analysis of solid samples.

A feature of the spark is that thermodynamic equilibrium does not have time to be established in its volume. Therefore, it is not entirely correct to speak about the temperature of the spark discharge as a whole. Nevertheless, it is possible to give an estimate of the effective temperature, which reaches a value of the order of 10,000°C. This is quite enough to excite the atoms of all currently known chemical elements.

The spark discharge is much more stable than the arc discharge, so the reproducibility of the results is higher.

Inductively coupled plasma (ISP). This is the most modern source of spectrum excitation, which has the best analytical capabilities and metrological characteristics in a number of parameters.

It is a plasma torch consisting of coaxially arranged quartz tubes. Highly pure argon is blown through them at high speed. The innermost flow is used as a carrier of the sample substance, the middle one is plasma-forming, and the outer one serves to cool the plasma. The argon plasma is initiated by a spark discharge and then stabilized by a high frequency inductor placed on top of the burner. In this case, a ring current of charged particles (ions and free electrons) of the plasma arises. The plasma temperature varies with the height of the burner and can reach 10,000°C.

The method of atomic emission spectroscopy using ICP is characterized by universality (most elements are excited at the plasma temperature), high sensitivity, good reproducibility, and a wide range of determined concentrations. The main factor hindering the wide application of this method in analytical practice is the high cost of equipment and consumables (high purity argon).

On fig. 9.1 shows a modern instrument for atomic emission spectral analysis with ICP as an excitation source.

Rice. 9.1.

Simultaneous measurement over the entire wavelength range ensures the highest accuracy and speed of analysis.

Spectra registration methods. Atomic emission spectroscopy uses single- and multichannel methods for recording spectra. To decompose the radiation of the sample into a spectrum, mono- and polychromators are used. As a rule, atomic spectra contain a large number of lines, so the use of high-resolution equipment is necessary. In the method of flame photometry, due to the small number of observed lines, light filters can be used instead of prism or diffraction monochromators.

Measurement of the intensity of spectral lines can be carried out visual, photochemical(photographic) and photovoltaic

ways. In the first case, the eye serves as the radiation receiver, in the second, a photographic emulsion, in the third, a photodetector (photocell, photomultiplier, photodiode, etc.). Each method has its advantages, disadvantages and optimal scope.

Visual methods of recording spectra are used for mass semi-quantitative steeloscopic and stylometric studies of the composition of materials, mainly metals. In the first case, a visual comparison is made between the intensities of the spectral lines of the element being determined and the nearby lines of the internal standard. Due to the peculiarities of the eye as a radiation receiver, with sufficient accuracy, it is only possible to either establish the equality of the intensities of neighboring lines, or to select the brightest line from the observed group.

The stylometric analysis differs from the stylosconic one by the possibility of controlled attenuation of the brighter line of the analytical pair. In addition, styliometers provide for the possibility of approaching the compared lines in the field of view. This makes it possible to more accurately estimate the ratio of the intensities of the analytical line and the comparison line.

The limit of detection of elements by a visual method is usually two orders of magnitude worse compared to other methods of recording spectra. The measurements themselves are rather tedious and undocumented.

However, the great advantages of the visual method are its simplicity, high performance and low equipment cost. It takes no more than 1 minute to determine one component. Therefore, the method is widely used for the purposes of express analysis in cases where high accuracy of the results is not required.

The most widely used in atomic emission spectral analysis is the photographic method of recording spectra. It is quite simple in execution technique and is publicly available. The main advantages of photographic recording are documented analysis, simultaneous recording of the entire spectrum, and low detection limits for many elements. In an automated version, this method acquires another advantage - a huge information content. No other methods are currently available to simultaneously determine up to 75 elements in one sample by analyzing several hundred spectral lines.

The properties of a photographic image depend on the total number of quanta absorbed by the photographic emulsion. This makes it possible to carry out analysis at a low signal level at the output of the system by increasing the exposure time. An important advantage of the method is the possibility of multiple statistical processing of spectra photographs.

With the photographic recording method, the intensity of a spectral line is determined by the blackening (optical density) of the image of this line on a photographic plate (photographic film). The main drawback of photographic materials is the non-linear dependence of blackening on illumination, as well as the wavelength of light, development time, temperature of the developer, its composition, and a number of other factors. Therefore, for each batch of photographic plates, it is necessary to experimentally determine characteristic curve, i.e. dependence of the amount of blackening S from the logarithm of illumination E S =f(gE). For this, a stepped attenuator is usually used, which is a quartz or glass plate with a set of translucent metal strips deposited on its surface, usually made of platinum, with different, but previously known transmittances. If a photographic plate is exposed through such an attenuator, areas with varying amounts of blackening will appear on it. By measuring the amount of blackening of the area and knowing the transmittance for each of them, it is possible to construct the characteristic curve of the photographic plate. A typical view of this curve is shown in Fig. 9.2.

Rice. 9.2.

L - blackening threshold; LV - underexposure area; sun- area of ​​normal blackening;

cd- overexposure area

The shape of the curve ns does not depend on the choice of illumination units and does not change if the illumination is replaced by the radiation intensity; therefore, it can be constructed by plotting the logarithms of the transmittances of the step attenuator along the abscissa axis.

The curve has a straight section sun(region of normal blackening), within which the contrast factor

takes a constant and maximum value. Therefore, the relative intensity of two spectral lines within the region of normal blackening can be found from the relations

Photometry of spectral lines and processing of the obtained data is one of the most laborious stages of atomic emission spectral analysis, which, moreover, is often accompanied by subjective errors. The solution to this problem is the automation on the basis of microprocessor technology of the processes of processing photographs of spectra.

For photoelectric recording, photocells, photomultiplier tubes (PMT) and photodiodes are used. In this case, the magnitude of the electrical signal is proportional to the intensity of the measured light flux. In this case, either a set of photodetectors is used, each of which registers the intensity of only its specific spectral line (multichannel devices), or the intensity of the spectral lines is successively measured by one photodetector when scanning the spectrum (single-channel devices).

Qualitative atomic emission analysis. The qualitative analysis is as follows:

• determination of the wavelengths of the lines in the spectrum of the sample;
• comparison of the results obtained with the data given in special tables and atlases, and the establishment of the nature of the elements in the sample.

The presence of an element in the sample is considered proven if at least four lines in the sample coincide in length with the tabular data for this element.

Length measurement, which is not very accurate, can be carried out on the scale of the instrument. The obtained spectrum is more often compared with the known spectrum, which is usually used as the spectrum of iron, which contains a large number of well-studied spectral lines. To do this, the spectrum of the sample and the spectrum of iron are photographed in parallel on one photographic plate under identical conditions. There are atlases in which the spectra of iron are given, indicating the position of the most characteristic lines of other elements, using which one can establish the nature of the elements in the sample (see work No. 34).

If the wavelengths of the lines are known, for example, in the spectrum of iron, between which there is a line with an unknown wavelength, the wavelength of this line can be calculated using the formula

where X x - wavelength of the determined line, X t X Y distance from the line with wavelength l 1 to the determined line; x 2- distance from the line with a wavelength l 2 to the determined line. This formula is only valid for a small range of wavelengths. The distance between the lines in the spectrum is usually measured using a measuring microscope.

Example 9.1. In the spectrum of the sample between the lines of iron X x = 304.266 nm and X 2 == 304.508 nm there is one more line. Calculate the wavelength of this line x x, if on the screen of the device it is 1.5 mm from the first line of iron, and 2.5 mm from the second.

Decision. We use the above formula:

If the spectrum of the sample is not too complex, elements in the sample can be identified by comparing the spectrum of the sample with the spectra of standards.

Methods of quantitative analysis. Quantitative spectral analysis uses the three standard method, the constant plot method, and the additive method.

Using three standard method the spectra of at least three standards (samples of known concentration) are photographed, then the spectra of the analyzed samples and a calibration graph is plotted in the coordinates AS- lg C".

Example 9.2. When analyzing the contact material for chromium by the method of three standards on an MF-2 microphotometer, blackening of 5 lines of a homologous pair was measured in the spectra of the standards and the test sample. Let's find the percentage of chromium C Cr according to the data from the table. 9.2.

Table 9.2

Data for example 9.2

Decision. The method of three standards uses the dependence of the difference S blackening of the lines of a homologous pair from the logarithm of the concentration of the element being determined. Under certain conditions, this dependence is close to linear. According to the readings of the measuring scale of the microphotometer, we find:

We determine the logarithms of concentrations: IgC, = -0.30; lgC2 = 0.09; lgC 3 = 0.62 and build a calibration graph in the coordinates AS- IgC" (Fig. 9.3).

Rice. 93.

We find D5 for the analyzed sample: D S x\u003d 0.61 - 0.25 \u003d 0.36, and according to the calibration graph we determine C l: lgC Cr = 0.35; C Cr = 2.24%.

Constant schedule method used for mass analysis of homogeneous samples. In this case, knowing the contrast of the photographic plates, they use once built a constant graph in the coordinates "D5 / y - IgC". When working in the area of ​​normal blackening, this will be equivalent to the coordinates "lg IJI- IgC. When working in the area of ​​underexposure, according to the characteristic curve of the photographic plate (5 = /(lg/)) for values ​​of 5 H and 5, lg/, and lg/ cp are found and a graph is plotted in the coordinates "lg/// p - IgC". In the area of ​​underexposure, to eliminate the curvature of the graph, it is necessary to subtract the blackening of the background of the photographic plate, measured near the line, from the blackening of the lines.

Example 9.3. To determine very small amounts of copper in a powdered material, the method of emission spectral analysis was used, which provides for three consecutive combustion of the sample in a DC arc and determination of the concentration from the intensity of the 3247 A copper line and from the constant graph "lgC - lg /" taking into account the background.

To construct the characteristic curve of a photographic plate with the spectra of a sample, the following data are available:

Decision. For three spectra, we calculate the difference between the copper lines and the background and find the average value:

Using the data given in the example condition, we build the characteristic curve of the photographic plate in the coordinates "D S-lg I"(Fig. 9.4).

From the characteristic curve for 5 cp = 1.48 we find lg/ = 1.38.

We build a calibration graph in the coordinates "lg / - IgC" (Fig. 9.5).

According to the calibration graph for lg / = 1.38 we find lgC= -3.74, which corresponds to the concentration of copper in the sample 1.8-10 4%.

Rice. 9.4.

Rice. 95.

Additive method used in the analysis of single samples of unknown composition, when there are special difficulties associated with the preparation of standards, the composition of which must be exactly identical to the composition of the sample (matrix effect). In this method, the analyzed sample is divided into parts and the element to be determined is introduced into each of them in a known concentration.

If the concentration of the mat element to be determined and the self-absorption effect can be neglected, then

In this case, one supplement is enough:

If a b 7^1 and I= as b, at least two additives are needed: ( C x + WITH () and (C x + From 2). After photographing and measuring the blackening of the lines on the photographic plate, a graph is plotted in the coordinates AS- lgС 7 ", where AS = 5 L - C p I = 1,2, - additive concentration. Extrapolating this graph to zero, one can find the value From x.

In addition to the graphical method, the calculation method is used, especially if the number of additives is large.

Example 9.4. Let us determine the content of niobium in the sample (%) by the method of additions according to the data in Table. 9.3 and 9.4 (TI - comparison line).

Table 9.3

Analytic blackening

Decision. According to the data given in the condition of the example, we build the characteristic curve of the photographic plate (Fig. 9.6).

Rice. 9.6.

According to the characteristic curve, using the blackening of the spectral lines for niobium and titanium, we find lg / Nb , lg / Tj , lg (/ N .,// Ti), / Nb // Ti) (Table 9.5).

Table 9.5

Calculations for example 9.4

Sample parts	Niobium concentration in the sample
Initial
With the first addition	C x + 0,2
With a second addition	C g + 0,6

We build a graph of the dependence "/ Nb // Ti - C forehead" (Fig.

Rice. 9.7.

The continuation of the graph until the intersection with the x-axis allows you to determine

intersection point coordinate: -0.12. Thus, the concentration of niobium

in sample C x is 0.12%.

Metrological characteristics and analytical possibilities of atomic emission spectroscopy. Sensitivity. The limit of detection in an atomic emission spectrum depends on the method of excitation of the spectrum and the nature of the element to be determined, and can change significantly with changes in the conditions of analysis. For easily excitable and easily ionizable elements (alkali and most alkaline earth metals), the best source of excitation of spectra is a flame. For most other elements, the highest sensitivity is achieved using inductively coupled plasma. The high limits of detection in a spark discharge are due to the fact that it is localized in a very small region of space. Accordingly, the amount of evaporated sample is also small.

Range of determined contents. The upper limit of the determined contents is determined mainly by the effect of self-absorption and the violation of the linearity of the calibration curve associated with it. Therefore, even when constructing a calibration graph in logarithmic coordinates, the range of determined contents is usually 2-3 orders of magnitude of concentrations. An exception is the method using ICP, for which the effect of self-absorption is very weak, and in connection with this, the range of linearity can reach 4–5 orders of magnitude.

Reproducibility. In atomic emission spectroscopy, the analytical signal is very sensitive to temperature fluctuations. Therefore, the reproducibility of the method is low. The use of the internal standard method can significantly improve this metrological indicator.

Selectivity mainly limited by the aliasing effect of spectral lines. Can be improved by increasing the resolution of the equipment.

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PHYSICAL EFFECT

Atomic emissionspectroscopy, nuclear power station or atomic emission spectral analysis - a set of elemental analysis methods based on the study of the emission spectra of free atoms and ions in the gas phase in the wavelength range of 150-800 nm.

Atomic emission spectral analysis is a method for determining the chemical composition of a substance from the emission spectrum of its atoms under the influence of an excitation source (arc, spark, flame, plasma).

Atoms are excited when one or more electrons move to a more distant energy level. In the normal state (unexcited) the atom has the lowest energy E 0 . An atom can be in an excited (unstable) state for a very short time (?10 -7 - 10 -8 sec) and always tends to occupy a normal unexcited state. In this case, the atom gives off excess energy in the form of photon radiation.

where E 2 , E 1 - the energy of the upper and lower levels; n - frequency; c is the speed of light; l is the radiation wavelength; h is Planck's constant.

To move an atom to a higher energy level, it needs to transfer energy called the excitation potential. The smallest energy necessary for detachment from an unexcited atom of its outer valence electron is the ionization potential (excitation energy).

Spectral line - radiation of any one wavelength, corresponding to a certain energy transition of an excited atom.

The intensity of the spectral line (I) is directly proportional to the number of excited particles (N *), because the excitation of atoms is of a thermal nature. Excited and unexcited atoms are in thermodynamic equilibrium with each other, which is described by the Boltzmann equation:

where N 0 is the number of unexcited atoms; g*, g 0 - static weights of excited and unexcited states of atoms; E - excitation energy; k - Boltzmann's constant; T is temperature.

Thus, at a constant temperature, N * is directly proportional to N 0, i.e. actually the total number of these atoms in the sample. The total number of atoms is directly proportional to the concentration (c) of the element in the sample.

That is, the intensity of the emission spectral line can be used as an analytical signal to determine the concentration of an element:

where a is a coefficient depending on the process conditions.

In AESA, the correct choice of atomization conditions and measurement of the analytical signal is of decisive importance, therefore, in real AESA conditions, the Lomakin-Scheibe formula is used:

where b is a constant coefficient depending on the energy transitions due to the emission of a given spectral line; determines the angle of inclination of the calibration graph of the controlled element.

Since the chemical composition of the samples is controlled over a wide range of concentrations, the Lomakin-Scheibe formula is used in logarithmic coordinates:

Graph 1 Calibration plot of the dependence of the intensity of the spectral line on the concentration of the element being determined

Graph 2 Graded characteristic for the determination of sulfur S in high chromium steels

PRINCIPAL DIAGRAM OF NPP CONDUCTION

Spectral analysis is based on the study of the structure of light that is emitted or absorbed by the analyzed substance. Let us consider the scheme of emission spectral analysis (Fig. 1). In order for a substance to emit light, it is necessary to transfer additional energy to it. The atoms and molecules of the analyte then pass into an excited state. Returning to their normal state, they give off excess energy in the form of light. The nature of the light emitted by solids or liquids usually depends very little on the chemical composition and therefore cannot be used for analysis. The radiation of gases has a completely different character. It is determined by the composition of the analyzed sample. In this regard, in the emission analysis, before excitation of a substance, it must be evaporated.

Fig 1 Schematic diagram of emission spectrum analysis: 1 -- Light source; 2 - lighting condenser; 4 -- spectral apparatus; 5 -- spectrum registration; 6 -- determination of the wavelength of spectral lines or bands; 7 -- qualitative analysis of the sample using tables and atlases; 8 -- determination of the intensity of lines or bands; 9 -- quantitative analysis of the sample according to the calibration curve

Evaporation and excitation is carried out in sources Sveta, into which the analyzed sample is introduced. As light sources, a high-temperature flame or various types of electric discharge in gases are used: an arc, a spark, etc. To obtain an electric discharge with the desired characteristics, generators.

High temperature (thousands and tens of thousands of degrees) in light sources leads to the disintegration of the molecules of most substances into atoms. Therefore, emission methods serve, as a rule, for atomic analysis and only very rarely for molecular analysis.

The radiation of the light source is the sum of the radiation of the atoms of all elements present in the sample. For analysis, it is necessary to isolate the radiation of each element. This is done with the help of optical devices - spectral devices. , in which light rays with different wavelengths are separated in space from each other. The radiation of a light source, decomposed into wavelengths, is called the spectrum.

The main parts of the spectral device (Fig. 2) are: entrance slit S, illuminated by the studied radiation; collimator lens O 1 , in the focal plane of which the entrance slit is located S; dispersing device D, working in parallel beams of rays; focusing lens O 2 , which creates in its focal surface R monochromatic images of the entrance slit, the totality of which forms the spectrum. As a dispersing element, as a rule, either prisms or diffraction gratings are used.

Fig. 2 Schematic optical diagram of a spectral device (l 1< л 2 <л 3)

Spectral devices are designed in such a way that the light vibrations of each wavelength entering the device form a single line. How many different waves were present in the radiation of the light source, so many lines are obtained in the spectral apparatus.

The atomic spectra of elements consist of individual lines, since there are only certain certain waves in the radiation of atoms (Fig. 3, a). In the radiation of hot solid or liquid bodies, there is light of any wavelength. Separate lines in the spectral apparatus merge with each other. Such radiation has a continuous spectrum (Fig. 3, in). In contrast to the line spectrum of atoms, the molecular emission spectra of substances that did not decompose at high temperature are striped (Fig. 3, b). Each band is formed by a large number of closely spaced lines.

Rice. 3 Types of spectra

Spectra types:

a- lined; b- striped; the individual lines that make up the band are visible; in--solid.

The darkest places in the spectrum correspond to the highest light intensity (negative image)

Light, decomposed into a spectrum in a spectral apparatus, can be viewed visually or recorded using photography or photoelectric devices. The design of the spectral apparatus depends on the method of recording the spectrum. For visual observation of spectra, spectroscopes are used - steeloscopes and steelometers. Spectra photography is carried out using spectrographs . Spectral devices - monochromators - allow you to select light of one wavelength, after which it can be registered using a photocell or other electrical light receiver.

In a qualitative analysis, it is necessary to determine which element emits one or another line in the spectrum of the analyzed sample. To do this, you need to find the wavelength of the line by its position in the spectrum, and then, using tables, determine its belonging to one or another element. Measuring microscopes, spectroprojectors, and other auxiliary devices are used to examine an enlarged image of the spectrum on a photographic plate and determine the wavelength.

The intensity of the spectral lines increases with the concentration of the element in the sample. Therefore, to conduct a quantitative analysis, it is necessary to find the intensity of one spectral line of the element being determined. The intensity of the line is measured either by its blackening in the photograph of the spectrum (spectrogram) or directly by the magnitude of the light flux coming out of the spectral apparatus. The amount of blackening of the lines on the spectrogram is determined on microphotometers.

The relationship between the intensity of the line in the spectrum and the concentration of the element in the analyzed sample is established using standards - samples similar to those analyzed, but with a precisely known chemical composition. This relationship is usually expressed in the form of calibration curves.

Below are the emission spectra of Fe and H:

Rice. 4.1 Emission spectrum of Fe

Rice. 4.2 Emission spectrum H

RESEARCH METHODOLOGY

The process of atomic emission spectral analysis consists of the following main parts:

1. Sample preparation (sample preparation)

2. Evaporation of the analyzed sample (if it is not gaseous);

3. Dissociation - atomization of its molecules;

4. Excitation of the radiation of atoms and ions of the elements of the sample;

5. Decomposition of the excited radiation into a spectrum;

6. Spectrum registration;

7. Identification of spectral lines - in order to establish the elemental composition of the sample (qualitative analysis);

8. Measurement of the intensity of the analytical lines of the elements of the sample to be quantified;

9. Finding the quantitative content of elements using pre-established calibration dependencies.

A sample of the test substance is introduced into the radiation source, where it evaporates, dissociates molecules, and excites the formed atoms (ions). The latter emit characteristic radiation, which enters the recording device of the spectral instrument.

With qualitative AESA, the spectra of samples are compared with the spectra of known elements given in the corresponding atlases and tables of spectral lines, and thus the elemental composition of the analyte is established. In quantitative analysis, the concentration of the desired element in the analyzed substance is determined by the dependence of the magnitude of the analytical signal (blackening density or optical density of the analytical line on the photographic plate; light flux to the photoelectric receiver) of the desired element on its content in the sample. This dependence is determined in a complicated way by many difficult-to-control factors (gross composition of samples, their structure, fineness, parameters of the spectrum excitation source, instability of recording devices, properties of photographic plates, etc.). Therefore, as a rule, to establish it, a set of samples for calibration is used, which, in terms of gross composition and structure, are as close as possible to the analyzed substance and contain known amounts of the elements to be determined. Such samples can be specially prepared metal alloys, mixtures of substances, solutions, including standard samples produced by industry. To eliminate the influence on the results of the analysis of the inevitable difference in the properties of the analyzed and standard samples, different methods are used; for example, they compare the spectral lines of the element being determined and the so-called comparison element, which is close in chemical and physical properties to the element being determined. When analyzing materials of the same type, one and the same calibration dependences can be used, which are periodically corrected according to verification samples.

The sensitivity and accuracy of the AESA depend mainly on the physical characteristics of the sources of excitation of the spectra - temperature, electron concentration, the residence time of atoms in the zone of excitation of the spectra, the stability of the source mode, etc. To solve a specific analytical problem, it is necessary to choose a suitable radiation source, to achieve optimization of its characteristics using various methods - the use of an inert atmosphere, the imposition of a magnetic field, the introduction of special substances that stabilize the discharge temperature, the degree of ionization of atoms, diffusion processes at an optimal level, etc. In view of the variety of mutually influencing factors, methods of mathematical planning of experiments are often used in this case.

The results below include drawings that illustrate how strongly the spectra of various elements differ from each other (in this example, these are aluminum, copper, tungsten and iron).

On the y-axis - intensity I in conventional units. The abscissa shows the wavelength l in nanometers, the spectral range is 172-441 nm. The spectra were taken on a spark spectrometer.:

Rice. 5.1 Emission spectrum AL

Rice. 5.1 Cu emission spectrum

Rice. 5.1 W alloy emission spectrum

Rice. 5.1 Fe emission spectrum

CLASSIFICATION OF AESA METHODS

After obtaining the spectrum, the next operation is its apolitical assessment, which can be carried out by an objective or subjective method. Objective methods can be divided into indirect and direct. The first group covers spectrographic, and the second - spectrometric methods. In the spectrographic method, the photographic emulsion makes it possible to obtain an intermediate characteristic of the line intensity, while the spectrometric method is based on direct measurement of the spectral line intensity using a photoelectric light detector. In the subjective evaluation method, the sensitive element is the human eye.

Spectrographicanalysis

The spectrographic method consists in photographing the IR spectrum of suitable plates or film using an appropriate spectrograph. The resulting spectrograms can be used for qualitative, semi-quantitative and quantitative analyses. When exciting and photographing the spectra of samples of various materials, it is necessary to strictly adhere to the relevant instructions. Organizational issues of the creation and operation of the spectrographic laboratory should also be discussed.

Spectrographic methods of spectral analysis are of particular importance. This is mainly due to the high sensitivity of the photographic emulsion and its ability to integrate light intensity, as well as the huge amount of information contained in the spectrum, and the ability to store this information for a long time. The instruments and other equipment required are relatively inexpensive, the cost of materials is low, and the method is simple and easy to standardize. Spectrographic spectral analysis is suitable for routine analysis and scientific research. Its disadvantage lies in the fact that, due to the complexity of photographic operations, it is not suitable for rapid analyzes, and its accuracy is lower than, for example, the accuracy of spectrometric or classical chemical analysis. This is not always the case when determining element traces. It can be hoped that spectrographic analysis will be greatly developed, especially in the field of processing the huge amount of useful information contained in the spectrum with the help of an automatic microphotometer connected to a computer.

Spectrometricanalysis

The spectrometric analytical method differs from the spectrographic method essentially only in the way the spectrum is measured. Whereas in spectrographic analysis the intensity of the spectrum is measured through an intermediate photographic step, spectrometric analysis is based on direct photometry of the intensity of the spectral lines. Direct measurement of the intensity has two practical advantages: due to the absence of a long processing operation of the photographed spectra and associated error sources, both the speed of analysis and the reproducibility of its results increase significantly. In spectrometric analysis, the operations of sampling, preparation, and excitation of spectra are identical to the corresponding operations of the spectrographic method. The same applies to all processes occurring during arousal, and spontaneous or artificially created effects. Therefore, they will not be discussed further here. The optical setup used in the spectrometric method, including the radiation source, its display, the entire dispersive system and spectrum acquisition, is almost identical to the spectrographic setup. However, a significant difference, which deserves separate discussion, lies in the method of supplying the light energy of the spectral lines to the photoelectric layer of the photomultiplier. The final operation of analysis, namely measurement, is completely different from the corresponding operation of the spectrographic method. Therefore, this stage of the analysis needs a detailed discussion.

Visualanalysis

The third group of emission spectral analysis methods includes visual methods, which differ from the spectrographic and spectrometric methods in the way the spectrum is estimated and, except in rare cases, in the region of the spectrum used. Spectrum Estimation Method subjective as opposed to the objective methods of the other two methods. In visual spectroscopy, the human eye is the receiver of light and uses the visible region of the spectrum from about 4000 to 7600 A*.

In the visual methods of spectral analysis, the preliminary preparation of samples and the excitation of their spectra essentially do not differ from similar operations of other methods of spectral analysis. At the same time, the decomposition of light into a spectrum is carried out exclusively with the help of a spectroscope. Finally, due to the subjectivity of the evaluation method, visual techniques differ significantly from spectrographic and especially spectrometric techniques. This also means that of the three methods of spectral analysis, the visual one has the least accuracy.

The detection limit of the visual method is relatively large. The most sensitive lines of elements, with the exception of alkali and alkaline earth, are in the ultraviolet region of the spectrum. Only relatively weak lines of the most important heavy metals are located in the visible region. Therefore, their limit of detection by the visual method is usually ten to a hundred times worse. Except in very rare cases, the visual method is not suitable for determining non-metallic elements, since their lines in the visible region are especially weak. In addition, the excitation of non-metallic elements requires special complex equipment and the intensity of the light source is not sufficient to evaluate the spectral lines with the naked eye.

In contrast to the disadvantages noted above, the great advantage of the visual method lies in its simplicity, speed and low cost. Working with the spectroscope is very easy. Although some training is required to evaluate the spectrum, simple analyzes can be learned quickly. The spectra can be assessed with the naked eye without the difficulties associated with indirect methods. This method is express: it usually takes no more than a minute to determine one component. The cost of relatively simple visual aids is low, and the cost of sample processing tool, counter electrode materials, and power is negligible. The techniques are so simple that after some training, analyzes can be performed by unskilled laboratory assistants. Due to the high rapidity of the method, labor costs per analysis are low. The economic efficiency of the method also increases due to the fact that the analysis can be carried out without destroying the analyzed sample and at the place where it is located. This means that with portable instruments it is possible to analyze intermediate products (eg metal rods), finished products (eg machine parts) or already installed products (eg overheated tubes of steam boilers) without sampling on site. Tools and time are also saved, organizational work is simplified and there is no need for destructive sampling methods.

The most important field of application of the visual method of spectral analysis is the control of metal alloys and mainly alloyed steels during their production for the purpose of sorting. The method is also used for the classification of metals or alloyed steels in the selection of valuable materials from scrap metal. In other areas, for example, in the analysis of dielectric materials, the visual method does not yet play a significant role. However, it is expected that after improvement it may find application in this and similar areas.

EXCITATION SOURCES OF THE SPECTRA

In the practice of atomic emission spectral analysis, flame, electric arcs of direct and alternating current, low- and high-voltage condensed spark, low-voltage pulsed discharge, various forms of glow gas discharge, etc. are used as sources of excitation of spectra. Over the past 10-15 years, various types of high-frequency discharges: high-frequency inductively coupled plasma (ICP) in an atmosphere of inert gases at atmospheric pressure, microwave discharge, etc.

1 Flame

The flame is used as an atomizer and a source of excitation of spectra in the method of flame photometry, and also as one of the main methods of atomization of substances in the method of atomic absorption analysis. The most commonly used flames are mixtures of air - acetylene (T = 2100-2400 K) and nitric oxide (I) - acetylene (T = 3000-3200 K), less often - flames of mixtures of air - propane (T = 2000-2200 K) and nitric oxide (I) - propane (T \u003d 3000 K).

The diagrams of the burners used in the flame photometry method are shown in fig. 1. The input of the analyzed liquid into the flame is usually carried out by its pneumatic spraying. There are mainly two types of atomizers used: angular and concentric, which operate due to the vacuum created above the opening of the atomizing capillary (or around it), the second end of which is immersed in the solution of the analyzed sample. The liquid flowing out of the capillary is sprayed with a jet of gas, forming an aerosol. The quality of the sprayer is evaluated by the ratio of the amount of liquid and gas ( M F /M D) consumed per unit of time.

The flame temperature provides a sufficiently low detection limit for elements whose energies, the excitation of resonance lines of which do not exceed 5 eV; their compounds are sufficiently atomized in the flame. The method of flame photometry is of particular importance for the determination of microquantities of compounds of alkali and alkaline earth metals, for which the limit of detection by this method is in the range of 0.0001-0.01 mg/l. The high spatiotemporal stability of the flames ensures good reproducibility of the results obtained by this method. When using continuous spraying solutions, the relative standard deviation characterizing the reproducibility is not at the level of 0.01 for contents exceeding the limit of detection by two orders of magnitude or more.

Rice. 6 Burners for atomic emission flame spectrometry: a) and b) conventional Mekker burner and improved burner: 1 - burner body; 2 - the surface on which the flame is formed; 3 - openings for the exit of combustible gases; 4 -- supply of a mixture of combustible gases and aerosol; 5 -- protrusion on the body of the burner with holes; in) combined burner with separation of the zones of evaporation - atomization and excitation of the spectra: 1 -- the main burner with a ledge and holes in it; 3 -- second additional burner with the same type or higher temperature flame; 4 - flame; 5 -- zone of registration of radiation; 6 - supply of a mixture of combustible gases to an additional burner; 7 -- supply of a mixture of combustible gases and aerosol to the main burner

The main limitations of the flame photometry method are: the need to transfer analyzed samples into a solution, a relatively high level of matrix effects, and, as a rule, single-element analysis.

Electric arc

DC electric arc

A direct current electric arc (Fig. 2) is a higher temperature source than a flame. The analyzed sample in crushed form is placed in a recess (channel) in the lower electrode, which, as a rule, is included as an anode in the arc circuit.

Rice. 7 DC arc as a source of spectrum excitation: a) DC arc supply circuit; b) volt-ampere characteristic of the DC arc discharge; in) diagram of the transfer of atoms from the channel of the carbon electrode: 1 - fraction of atoms involved in the formation of an analytical signal ( 1a- removal in a free state, 1b-- removal in the bound state in the condensed phase); 2 -- release of substance in addition to the excitation zone; 3a, 3b-- diffusion into the walls and bottom, respectively; 4a, 4b -- the transition of a substance into the excitation zone in the form of atoms or compounds from the walls and bottom of the electrode

The temperature of the arc plasma depends on the material of the electrodes and the ionization potential of the gas in the interelectrode gap. The highest plasma temperature (~7000 K) is achieved when carbon electrodes are used. For an arc between copper electrodes, it is? 5000 K. The introduction of salts of alkaline elements (for example, potassium) reduces the temperature of the arc plasma to 4000 K.

Under the action of the arc, the anode end is heated to approximately 3500 K (for carbon electrodes), which ensures the evaporation of solid samples placed in the anode crater. However, the temperature of the electrode in the direction from the end drops very quickly and is only ≈1000 K already at a distance of 10 mm. By giving the electrode a special shape, it is possible to reduce heat removal and thereby increase the electrode temperature to some extent.

In a DC carbon arc, the spectra of almost all elements are excited, with the exception of some gases and non-metals, which are characterized by high excitation potentials. Compared to flame emission or absorption measurements, arc discharge provides about an order of magnitude lower element detection limit, as well as a significant reduction in matrix effects.

The arc discharge is unstable, one of the reasons for this is the continuous movement of the cathode spot, which actually provides the thermionic emission necessary to maintain the discharge. To eliminate the instability of the arc, a large ballast resistance is included in its circuit. R. The current flowing through the arc, according to Ohm's law

Here U- voltage of the source supplying the arc; r- resistance of the arc gap.

The more ballast resistance R, the smaller the influence of fluctuations r to a change in the electric current of the arc. For the same reason, it is beneficial to increase the arc supply voltage (you can take more R). In modern generators, the arc supply voltage is usually 350 V. The arc current is usually in the range of 6-10 A. To evaporate refractory materials (for example, Al 2 O 3), an increase in current to 25-30 A is required. allow to stabilize the arc current at the level of 25 A with fluctuations of no more than 1% when the supply voltage changes within 200-240 V, and the use of magnetic amplifiers as a regulating element makes it possible to increase the efficiency of the arc generator up to 90% .

To improve the conditions for excitation of the spectra, one uses controlled atmosphere(for example, argon or other gaseous media), stabilization of the position of the plasma in space by a magnetic field (in particular, a rotating one) or a gas flow. The use of a controlled atmosphere makes it possible to get rid of the cyanide bands observed in the region of 340–420 nm and overlapping many sensitive lines of various elements.

AC electric arc

The arc discharge can also be powered by alternating current, but such a discharge cannot exist independently. When the current direction changes, the electrodes cool down quickly, thermionic emission stops, the arc gap deionizes and the discharge goes out, therefore, special ignition devices are used to maintain the arc burning: the arc gap is pierced by a high-frequency, high-voltage, but low-power pulse (Fig. 3).

Rice. eight Schematic of a low voltage activated AC arc: I -- main contour; II-- auxiliary circuit; R-- arc supply rheostat; BUT - ammeter; d -- working span of the arc; L-- secondary coil of a high-frequency transformer; With-- blocking capacitor (0.5-2 uF); Tr-- step-up transformer; La --primary coil of high-frequency transformer; Sa-- activator capacitor (3000 uF); RTp- resistance of the activator; da -- bit gap of the activator

The scheme of such an arc can be divided into two parts: the main and auxiliary. The main part of the circuit looks exactly the same as for the DC arc, except for the shunt capacitor. With, preventing the penetration of high-frequency currents into the network.

In the activator, a step-up transformer (120/260/3000 V, 25 W) creates a voltage of ~ 3000 V on the secondary winding and charges the capacitor Ca. At the moment of breakdown of the auxiliary spark gap yes in circuit consisting of a coil La, condenser Ca and arrester da, high frequency oscillations appear. As a result, at the ends of the second (high-voltage) coil L an EMF of about 6000 V occurs, penetrating the working gap d. These breakdowns serve to periodically ignite the arc fed through the main circuit.

The stability of the electrical and optical parameters of the AC arc depends on the stability of the voltage at which the breakdown occurs. Ignition control by breakdown of the auxiliary gap does not provide the required accuracy due to oxidation and other changes in the working surfaces of the spark gap over time. More stable operation of the arc can be achieved by adjusting the ignition phase of the discharge using electronic devices. Such control schemes are used in most modern generators.

To some extent, the pulsed nature of the AC arc leads to the fact that the discharge temperature becomes somewhat higher than in the DC arc, and measurements of the intensities of spectral lines are characterized by better reproducibility. At the same time, the control scheme can be configured in such a way that the breakdown of the gap is carried out not every half-cycle, but after one, two, four, etc. This allows you to control the heating of the electrodes, which may be necessary, for example, when analyzing low-melting alloys.

To reduce the limits of detection of elements and improve the reproducibility of the analysis results when working with arc discharges, the addition of certain reagents to the analyzed samples is widely used in order to initiate various types of thermochemical reactions directly in the channels of the arc electrodes. These reactions make it possible to convert the impurities to be determined into highly volatile compounds, and the matrix elements interfering with the determination of impurities into non-volatile compounds.

Arc in the variant of spillage

In addition to the traditional version of the arc with vertical electrodes, when analyzing powder samples (for example, ores and minerals), an arc is used in the so-called version spills (blowing) when a finely dispersed sample does not evaporate from the carbon electrode channel, but wakes up (is blown) through the arc discharge plasma between two (or three - with a three-phase power supply) horizontally located carbon electrodes.

Rice. 9 Schematic diagram of the introduction of a powder sample into an arc discharge by the method of spilling - blowing: 1 -- powder sample in a vibrating funnel; 2 - arc electrodes; 3 -- cooling and plasma-forming air flows; 4 -- cylindrical air duct; 5 - arc plasma; 6 -- a window in the air duct for observing radiation from the working area of ​​the arc plasma

The design and principle of operation of such an arc are shown in Fig. 4. In terms of parameters and characteristics, a horizontal arc differs little from a vertical one, however, due to the fact that the sample is introduced into the arc by a gas flow (usually air), it stabilizes the shape and position of the arc plasma, which in itself already helps to reduce random errors of analysis in terms of compared with a conventional spatially unstabilized arc between vertical electrodes. In addition, with uniform injection of powders, the composition of the arc cloud remains unchanged over time, therefore, the main parameters of the arc plasma (density of atoms and electrons, temperature) also remain constant, which greatly simplifies the analysis. The main problems of analysis by the injection method are related to the incomplete evaporation of powder particles due to the short duration of their stay in the plasma (3*10-3-5*10-3 s), which determines the dependence of the intensity of the spectral lines on the size and composition of the particles of powder samples.

Spark. low voltage spark

An increase in the capacitance of the shunt capacitor leads to the fact that the energy stored in it will play a significant role in the overall balance of the discharge. This type of discharge is called a low-voltage spark. Depending on the parameters of the low-voltage spark circuit, various discharge modes can be obtained: oscillatory ( CR 2 /4L<1), критический (CR 2 /4L>1), aperiodic ( CR 2 /4L?1).

The voltage on the capacitors of the discharge circuit usually varies in the range of 450-1000 V. By changing the capacitance of the capacitors, the resistance of the rheostats in the power circuit and the inductance of the secondary winding of the transformer, it is possible to adjust the ratio between the current strength of the discharge of the capacitors and the strength of the current passing through the power circuit, and thereby smoothly change the discharge temperature in the desired direction (from soft arc mode to pure spark mode). Modern electronic means make it possible to stabilize the energy of single pulses with an accuracy of no worse than 0.1%.

high voltage spark

In the spectral analysis of metals and alloys, a high-voltage condensed spark is most often used as a light source (Fig. 5). The step-up transformer charges the capacitor With up to a voltage of 10-15 kV. The voltage value is determined by the resistance of the auxiliary gap AT, which in turn is always chosen with a large working gap resistance BUT. At the moment of breakdown of the auxiliary gap, a breakdown of the working gap also occurs simultaneously, the capacitor With discharged and then charged. Depending on the circuit parameters and the gap deionization rate, the next breakdown can occur either in the same or in another half cycle. The simplicity and reliability of this scheme ensured its successful operation.

Rice. 10 Scheme of a controlled condensed high-voltage spark: T-- step-up transformer for 15000 V; With- condenser; L-- variable inductance; r-- blocking resistance; BUT- working interval; AT-- constant auxiliary gap; R-- adjustable resistance

At the moment of breakdown in a narrow spark channel, excitation occurs, as well as the emission of atoms and molecules of nitrogen and oxygen in the air; this is useless and even interfering radiation (background). However, its duration is short (10-8 s). At the next moment, the current (up to 50 A) passing through the channel heats up a small area (0.2 mm) of the electrode. The current density reaches 10 4 A/cm 2 , and the electrode material is ejected into the discharge gap in the form of a torch of hot vapor, and, as a rule, not along the spark channel, but at some random angle to it.

Each new breakdown affects different parts of the sample surface, and after spattering, during the entire selected exposure time, a spattering spot with a diameter of up to 3-5 mm, but of insignificant depth (when working with a carbon counter electrode, only 20-40 microns) appears on the sample. The total amount of the solid sample evaporating during exposure is very small: for example, for steels it is usually about 3 mg.

The ejected vapor torch has a temperature of about 10,000 K, i.e. sufficient not only to excite the spectra of metals, but also non-metals and ions. The temperature directly at the beginning of the spark reaches 30,000-40,000 K.

High frequency inductively coupled plasma

spectral atomic emission plasma

Owing to the emergence of a new method of excitation of spectra using a source of high-frequency inductively coupled plasma (ICP) operating at atmospheric pressure, there has been a sharp leap in the development of the physics, technology, and practice of atomic emission spectral analysis. This source is a kind of electrodeless high-frequency discharge maintained in a special burner, consisting of concentrically arranged three (rarely two) quartz tubes (Fig. 6). An external (cooling) gas flow (argon or molecular gas) is fed into the gap between the outer and intermediate tubes, an intermediate flow (argon only) is supplied through the middle tube, and the aerosol of the analyzed solution is transported into the plasma through the central tube. The open end of the burner is surrounded by a water-cooled induction coil connected to an RF generator. To obtain plasma, RF generators are used with a power consumption of 1.5-5 kW and an operating frequency in the range from 27 to 50 MHz.

Rice. 11 Scheme of a burner for a high-frequency induction discharge: 1 -- analytical zone; 2 -- zone of primary radiation; 3 -- discharge zone (skin layer); 4 -- central channel (preheating zone); 5 -- inductor; 6 -- a protective tube that prevents breakdown on the inductor (installed only on short burners); 7, 8, 9 -- outer, intermediate, central tubes respectively

To excite the discharge, preliminary ionization of the gas is necessary, since the voltage across the inductor is much less than the breakdown voltage of the working gas. For this purpose, a high-voltage spark (Tesla coil) is most often used. In an ionized gas, a discharge occurs, fed by a magnetic field. The high frequency current flowing through the solenoid coil creates an alternating magnetic field. Under its influence, a vortex electric field is induced inside the coil. The vortex electric current heats and ionizes the portions of gas coming from below due to the Joule heat. Conductive plasma is analogous to a short-circuited secondary winding of a transformer, the magnetic field of which compresses the ring current into a torus (skin effect).

The argon flow supplied to the gap between the intermediate and outer tubes, on the one hand, serves as a plasma-forming gas, and on the other hand, squeezes the hot plasma from the burner walls, protecting them from overheating and destruction. The aerosol of the analyzed sample propagates along the central channel of the discharge, practically without touching the electrically conductive skin layer and without affecting its characteristics; This is one of the main features of the ICP discharge, which distinguishes it, for example, from arc plasmatrons.

Usually, an aerosol is injected into the plasma, formed by a solution of the sample in an aqueous or organic solvent. Along with this, samples are introduced in the form of condensates formed during sample evaporation in an electrothermal atomizer, arc, spark, laser torch plasma, as well as in the form of fine powders suspended in a gas or liquid flow. Various designs of pneumatic atomizers (Meinhard concentric atomizer, angle atomizers, Babington atomizer, Hildebrand mesh atomizer, etc.), as well as ultrasonic atomizers, are used to introduce liquid samples. All types of nebulizers use a forced supply of the sample solution using a peristaltic pump.

In ultrasonic atomizers, atomization occurs due to the energy of acoustic vibrations, and the gas flow serves only to transfer the aerosol to the burner. These nebulizers produce a fine aerosol with a narrow particle size distribution. The efficiency of their generation is at least 10-20 times greater than that of pneumatic nebulizers, which makes it possible to obtain a better signal-to-background ratio and lower the detection limit.

The following unconditional advantages of the ICP source in relation to the problems of atomic emission spectral analysis (AESA) can be singled out:

1. Due to the possibility of effective excitation of both easily and hardly excitable lines, ICP is one of the most versatile light sources in which almost all elements of the periodic system can be determined (detected). ICP is the most universal source not only in terms of the number of elements to be determined, but also in terms of the type of compounds containing these elements;

2. ICP can analyze both large masses of solutions, feeding them into the plasma torch in a continuous flow, and microvolumes (on the order of hundreds of microliters) when they are pulsed into the carrier gas and spectra are pulsed;

3. The range of determined concentrations for most elements is 4-5 orders of magnitude, i.e. in ICP, it is possible to determine both low and medium, and high concentrations of one or another element, which is difficult for other sources of spectrum excitation. Calibration graphs for many elements are rectilinear, parallel to each other and have an inclination angle of about 45°, which simplifies calibration and reduces the likelihood of systematic analysis errors;

4. Due to the high excitation efficiency and low background, the limits of detection for most elements are 1-2 orders of magnitude lower than in other sources of spectrum excitation. The average detection limit in the analysis of solutions for all elements is approximately 0.01 mg/l, decreasing for some of them to 0.001-0.0001 mg/l;

5. With the stabilization and optimization of all operating conditions, the ICP flame has good spatial and temporal stability, which ensures high instrumental reproducibility of analytical signals, sometimes at the level of 0.5-1%.

The disadvantages of the ICP spectrometry method include the relatively high cost of operating spectrometers, associated with a high consumption of argon (15–20 l/min). Determination of trace metal contents near the detection limit is complicated by the presence in the spectrum of molecular bands -NO and -OH in the range of 200-260 and 280-340 nm, which appear at the periphery of the discharge, at the point of its contact with the atmosphere. To reduce the intensity of these bands, burners with an outer tube extended by 40–50 mm with a cut-out window for radiation output are used.

The ICP discharge is characterized by very developed spectra, with a large number of lines belonging to atoms, as well as singly and doubly charged ions. In this regard, the use of this excitation source is complicated by the effects of spectral interference, which leads to higher requirements for the resolving power of spectral instruments. Due to the lower brightness of the source, the role of scattered light in the device increases.

Spectral analysis methods are simple, easy to mechanize and automate, i.e. they are suitable for mass analyses. When using special methods, the limits of detection of individual elements, including some non-metals, are extremely low, which makes these methods suitable for determining trace amounts of impurities. These methods are practically non-destructive since only small amounts of sample material are required for analysis.

The accuracy of the spectral analysis, in general, satisfies the practical requirements in most cases of determining impurities and components. The cost of spectral analysis is low, although the initial investment is quite high. However, the latter quickly pay off due to the high productivity of the method and low requirements for materials and maintenance personnel.

Spectral analysis is not suitable for determining the types of relationships between elements. Like all instrumental methods of analysis, quantitative spectral analysis is based on a comparative study of the analyzed sample and standard samples of known composition.

Spectral analysis can be considered as a method of instrumental research that has found the greatest application. However, this method cannot fully satisfy the various analytical requirements that arise in practice. Thus, spectral analysis is only one laboratory method in a number of other research methods pursuing different goals. With reasonable coordination, different methods can perfectly complement each other and jointly contribute to their overall development.

In order to choose from the methods of spectral analysis the one that is most suitable for a given task, and to obtain the correct results by the chosen methods, appropriate theoretical and practical knowledge is required, very careful and accurate work.

Sampling must be done with the utmost care. Due to the high sensitivity of spectral release, inferences about the chemical composition of very large batches of material must often be made from the analysis of small amounts of a sample. Contamination of the analyzed sample can significantly distort the results of the analysis. Appropriate physical or chemical treatment of samples, such as fusion, dissolution or pre-enrichment, can often be very helpful.

To excite spectra in different methods, substances are required in different physical states or in the form of different chemical compounds. The throughput of an analysis can have a decisive influence on the selection of the most suitable radiation sources.

The ratio of the intensities of the lines of an analytical pair, even for the most thorough sampling method and when using the most suitable radiation source, largely depends on the external physical and chemical parameters (experimental conditions) specified by the analysis method and changing during excitation. Knowledge of theoretical correlations and practical conclusions from them is of great importance for the full realization of the analytical capabilities of the method.

The excited emission spectrum of the sample is recorded using a spectrograph, spectrometer or spectroscope. Therefore, methods for estimating spectra in spectral analysis can be divided into three groups.

In spectrographic qualitative analysis, a conclusion about the nature of the elements in the analyzed sample can be made on the basis of the wavelength of the spectral lines. In quantitative analysis, the blackening of lines in the general case serves as a measure of their intensity and, consequently, the desired quantitative composition of the sample.

The spectrometric method, in which the intensity of the lines is usually determined using a photomultiplier and measuring electronic equipment, refers to objective methods of quantitative analysis. This method of intensity measurement is more accurate and faster than the spectrographic one, but it requires expensive and difficult-to-maintain equipment.

Spectral analysis instruments for visual spectroscopy are relatively cheap and analyzes are fast. However, these methods are based solely on subjective ways of measuring the intensity of lines. Therefore, the results obtained are always semi-quantitative.

To achieve higher detection sensitivity, reproducibility and accuracy, it is necessary to process the measurement results by methods of mathematical statistics.

When performing spectral analysis, tables containing the corresponding physical constants and spectroscopic constants of the elements and their most important compounds, as well as tables for auxiliary calculations and work instructions necessary for qualitative and quantitative determinations, are of great help.

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Non-state non-profit educational institution of secondary vocational education "Pokrovsky Mining College"

Test

Atomic emission spectral analysis

Completed:

group student

"Lab Analyst"

Profession: OK16-94

Chemical analysis laboratory assistant

Introduction

2. Atomizers

3 Flame processes

5. Spectrographic analysis

6. Spectrometric analysis

7. Visual analysis

Conclusion

Bibliography

Introduction

The purpose of practical emission spectrum analysis is to detect qualitatively, semi-quantitatively or accurately quantify elements in the analyte.

Spectral analysis methods, as a rule, are simple, rapid, easy to mechanize and automate, i.e., they are suitable for routine mass analyses. When using special methods, the limits of detection of individual elements, including some non-metals, are extremely low, which makes these methods suitable for determining trace amounts of impurities. These methods, except when only a small amount of sample is available, are practically non-destructive, since only small amounts of sample material are required for analysis.

The accuracy of spectral analysis, in general, satisfies practical requirements in most cases of determining impurities and components, with the exception of determining high concentrations of the main components of alloys. The cost of spectral analysis is low, although the initial investment is quite high. However, the latter quickly pay off due to the high productivity of the method and low requirements for materials and maintenance personnel.

Goals of the work:

1. familiarization with the theory of atomic emission spectral analysis;

2. learn to understand the main characteristics of the APP equipment;

3. study of AESA methods;

1. Atomic emission spectral analysis (AESA)

Methods of analysis based on the measurement of any radiation by the substance being determined are called emission methods. This group of methods is based on measuring the wavelength of radiation and its intensity.

The method of atomic emission spectroscopy is based on the thermal excitation of free atoms or monatomic ions and registration of the optical emission spectrum of the excited atoms.

To obtain the emission spectra of the elements contained in the sample, the solution to be analyzed is brought into the flame. The flame radiation enters the monochromator, where it is decomposed into individual spectral lines. With a simplified application of the method, a certain line is highlighted with a light filter. The intensity of the selected lines, which are characteristic for the element being determined, is recorded using a photocell or photomultiplier connected to the measuring device. Qualitative analysis is carried out by the position of the lines in the spectrum, and the intensity of the spectral line characterizes the amount of substance.

The radiation intensity is directly proportional to the number of excited particles N*. Since the excitation of atoms is of a thermal nature, the excited and unexcited atoms are in thermodynamic equilibrium with each other, the position of which is described by the Boltzmann distribution law (1):

where N 0 is the number of unexcited atoms;

g* and g 0 are the statistical weights of the excited and unexcited states; E - excitation energy;

k - Boltzmann's constant;

T is the absolute temperature.

Thus, at a constant temperature, the number of excited particles is directly proportional to the number of unexcited particles, i.e. in fact, the total number of these atoms N in the atomizer (since under real conditions of atomic emission analysis, the fraction of excited particles is very small: N*<< N 0). Последнее, в свою очередь, при заданных условиях атомизации, определяемых конструкцией и режимом работы прибора и рядом других факторов), пропорционально концентрации определяемого элемента в пробе С. Поэтому между интенсивностью испускания и концентрацией определяемого элемента существует прямо пропорциональная зависимость:

Thus, the intensity of the emission spectral line can be used as an analytical signal to determine the concentration of an element. The coefficient a in equation (2) is a purely empirical value, depending on the process conditions. Therefore, in NPP, the correct choice of atomization conditions and measurement of the analytical signal, including calibration according to reference samples, is of decisive importance.

The method is widely used for analytical purposes in medical, biological, geological, agricultural laboratories.

emission spectral atomization photometer

2. Atomizers

The main types of atomization and excitation sources are given in Table 1.

Table 1

Atomization source type	T, ºC	Sample status	C min, % mass	Relates std. rejected
flame	1500 - 3000	solution		0,01 – 0,05
electric arc	3000- 7000	hard		01 – 0,2
electric spark	10000 -12000	hard		0,05 – 0,10
inductively coupled	6000 - 10000	solution		0,01 – 0,05

The most important characteristic of any atomizer is its temperature. The physicochemical state of the analyzed substance and, consequently, the magnitude of the analytical signal and the metrological characteristics of the technique depend on the temperature.

Flame. The flame version of the method is based on the fact that the analyte in the form of an aerosol, together with the solvent used, enters the flame of a gas burner. In a flame with the analyzed substance, a number of reactions occur and radiation appears, which is characteristic only for the substance under study and is in this case an analytical signal.

The diagrams of the burners used in the flame photometry method are shown in fig. 1. The input of the analyzed liquid into the flame is usually carried out by its pneumatic spraying. There are mainly two types of atomizers used: angular and concentric, which operate due to the vacuum created above the opening of the atomizing capillary (or around it), the second end of which is immersed in the solution of the analyzed sample. The liquid flowing out of the capillary is sprayed with a jet of gas, forming an aerosol. The quality of the sprayer is evaluated by the ratio of the amount of liquid and gas (M W / M G) consumed per unit time.

Rice. 1. Torches for atomic emission flame spectrometry:

a) and b) a conventional Mecker burner and an improved burner: 1 - burner body; 2 - the surface on which the flame is formed; 3 - openings for the exit of combustible gases; 4 - supply of a mixture of combustible gases and aerosol; 5 - protrusion on the burner body with holes; c) combined burner with separation of the zones of evaporation - atomization and excitation of spectra: 1 - main burner with a ledge and holes in it; 3 - second additional burner with the same type or higher temperature flame; 4 - flame; 5 - zone of registration of radiation; 6 - supply of a mixture of combustible gases to an additional burner; 7 - supply of a mixture of combustible gases and aerosol to the main burner.

For the formation of a flame, a gas mixture is prepared, consisting of a combustible gas and an oxidizing gas. The choice of components of one or another gas mixture is determined, first of all, by the required flame temperature.

Table 2 contains information about the temperatures of various tribes in atomic emission analysis and their main characteristics.

Table 2 Characteristics of tribes used in atomic emission analysis

Composition of the mixture		TºC
combustible gas	Oxidizing agent
methane CH4	Air	1700 -1900
hydrogen H2	Air	2000-2100
acetylene C 2 H 2	Air	2100-2400
acetylene C 2 H 2		2600-2800
acetylene C 2 H 2		3050-3150

There are certain analytical characteristics of the flame. The flame, of course, must be stable, safe, and the cost of components to maintain it must be low; it must have a relatively high temperature and a slow propagation velocity, which increases the efficiency of desolvation and vapor production, and results in large emission, absorption, or fluorescence signals. In addition, the flame must provide a reducing atmosphere. Many metals in a flame tend to form stable oxides. These oxides are refractory and difficult to dissociate at normal temperatures in a flame. To increase the degree of formation of free atoms, they must be restored. Reduction can be achieved in almost any flame if the combustible gas flow rate is set to a greater than required combustion stoichiometry. Such a flame is called enriched. The rich flames produced by hydrocarbon fuels such as acetylene provide an excellent reducing atmosphere due to the large amount of carbon-containing radical particles.

The flame is the lowest temperature source of atomization and excitation used in nuclear power plants. The temperatures reached in the flame are optimal for determining only the most easily atomized and excitable elements - alkali and alkaline earth metals. For them, the method of flame photometry is one of the most sensitive - up to 10 -7% of the mass. For most other elements, the limits of definition are several orders of magnitude higher. An important advantage of the flame as a source of atomization is its high stability and the associated good reproducibility of the measurement results (S r - 0.01-0.05).

The choice of the required flame temperature depends on the individual properties of the substances to be determined.

If, for example, easily excited substances (alkali metals) are to be determined, the flame temperature can be quite low.

Electric arc. In nuclear power plants, arc discharges of direct and alternating current are used. An electric discharge is passed between a pair of electrodes (usually carbon). In this case, a sample in the solid state is placed in the recess of one of the electrodes. The temperature of the arc discharge is 3000 - 7000 ºC. Such temperatures are sufficient for the atomization and excitation of most elements, except for the most difficultly excitable non-metals - halogens. Therefore, for a large number of elements, the limits of detection in an arc discharge are lower than in a flame, and are - 10 -4 - 10 -2 mass. %. Arc atomizers, unlike flame atomizers, do not have a high stability of operation, therefore, the reproducibility of the results is not high and amounts to Sr - 0.1-0.2. Therefore, one of the main areas of application of arc atomizers is qualitative analysis.

Electric spark. The spark atomizer has the same structure as the arc atomizer and is intended primarily for the analysis of solid samples at a qualitative level.

Inductively coupled plasma (ICP). The most modern source of atomization with the best analytical capabilities and metrological characteristics. The inductively coupled plasma atomizer is an argon plasma torch that is initiated by a spark charge and stabilized by a high frequency induction coil. The temperature of the argon plasma varies along the height of the burner and is 6000 - 10000 ºC. At such high temperatures, most elements are excited. The sensitivity of the method is 10 -8 - 10 -2 mass. % depending on the element. The reproducibility of the characteristics of the argon burner is high, which makes it possible to carry out quantitative analysis in a wide concentration range with a reproducibility of S r - 0.01-0.05. The main factor hindering the use of ICP NPPs is the high cost of equipment and consumables, in particular, high-purity argon, whose consumption during analysis is 10–30 l/min.

Rice. 6. Scheme of a burner for a high-frequency induction discharge:

1 - analytical zone; 2 - zone of primary radiation; 3 - discharge zone (skin layer); 4 - central channel (preheating zone); 5 - inductor; 6 - protective tube preventing breakdown on the inductor (installed only on short burners); 7, 8, 9 - outer, intermediate, central tubes, respectively

3. Flame processes

The analyzed substance MX in the form of an aerosol enters the flame and undergoes a number of transformations there:

MX (solution) ↔ MX (solid) ↔ MX (gas) ↔ M + X ↔ M + + X ↔ ...

M + + hν (M +)*

M* - excited state of the determined element M.

In the first stage, the solvent used evaporates and the molecular forms of previously dissolved substances in the crystalline state are formed. Then the process of disintegration of the molecules of the analyzed substances takes place. At sufficiently low temperatures, the molecules break down into atoms, at higher temperatures the ionization of the resulting atoms can occur, and at very high temperatures, bare nuclei and electron gas can form.

At the stage of atomization, atomic particles are excited by collision with each other or by absorption of radiation quanta.

Excitation is the transition of some electrons of an atom to a higher energy level.

In an excited state, atoms do not live long (10 -5 - 10 -8 sec), then they return to their original state, while emitting a quantum of energy. This quantum of energy emitted by an excited atom is the analytical signal in a nuclear power plant.

The line intensity in the emission spectrum can be calculated from the equation:

I v app. = hν 12 A 12 N 1

where h is Planck's constant,

ν 12 is the frequency of the transition between the states of the atom 1 and 2, which is related to the wavelength by the relation: νλ = c (c is the speed of light),

A 12 is the Einstein coefficient, which determines the probability of this transition,

N 1 is the number of atoms in state 1.

In the flame, in addition to the main processes noted above, some undesirable processes also occur, leading to the appearance of interference that interferes with the determination.

The most typical interferences are classified as follows:

Interference in the formation of atomic vapor

Spectral interference

Ionization interference.

Interferences in the formation of atomic vapor are observed in cases where some component of the sample affects the rate of evaporation of particles containing the analyte. The source of such interference can be a chemical reaction that affects the evaporation of solid particles, or a physical process during which the evaporation of the main components of the sample affects the formation of a pair of atoms (molecules) of the analytes.

An example of such influence is the determination of calcium in the presence of phosphate ions. It has been found that a calcium solution containing phosphate ions gives a lower signal in the flame than a calcium solution of the same concentration, but in the absence of phosphate ions.

It is assumed that this phenomenon is due to the formation of a stoichiometric compound between calcium and phosphate, which evaporates more slowly than calcium in the absence of phosphate ions.

Evidence for this assumption is that the degree to which phosphate suppresses the calcium signal is greatest at points located in the lower part of the flame outside the mediocre edge of the burner. If this signal is measured at the top of the flame, where the calcium-containing particles have more time to evaporate, then the magnitude of the signal increases as more of the calcium atoms that have been bound to the phosphate ions are released.

The interference caused by phosphate ions can be minimized not only by measuring the magnitude of the signal at the top of the flame, but also by other means.

Thus, the use of more advanced nebulizer and burner designs makes it possible to obtain a very fine aerosol, which easily forms, after evaporation of the solvent, the smallest particles of the analyte, which require much less time to evaporate, and the interference from the presence of phosphate ions is reduced.

It is also possible to increase the particle evaporation rate by increasing the temperature of the flame used.

Interference with the formation of atomic vapor can be minimized or eliminated entirely by the use of special substances called "release agents". These substances promote the release of calcium atoms from slowly evaporating calcium-containing particles.

For example, when large amounts of lanthanum ions are added to an analyzed solution containing calcium ions and phosphate ions, calcium atomization increases as a result of the fact that lanthanum ions predominantly bind to phosphate ions.

Complexing agents, for example, ethylenediaminetetraacetic acid, can act as releasing agents, the addition of which to the analyzed solution prevents the formation of a calcium compound with phosphate ions.

Another type of releasing agents is able to form a matrix in which calcium and phosphate can be dispersed. Such particles in the flame decompose very quickly and turn into steam. For example, if a large amount of glucose is added to a solution containing phosphate and alkaline earth elements, then after evaporation of the solvent, the particles will consist mainly of glucose in which calcium and phosphate ions are distributed. When such particles decompose in a flame, the particles of calcium with phosphate are very small and easily turn into steam.

The second undesirable process that takes place in the flame during the formation of atomic vapor is the formation of metal monoxides FeO, CaO, since oxygen is present in the composition of the combustible gas):

In this case, monoxides can also be excited and emit light, but in a different region of wavelengths. Eliminate this process by increasing the temperature of the flame.

The third undesirable process that occurs in the flame during the formation of atomic vapor is the formation of MeC carbides (carbon is present in the combustible gas). To suppress this process, the necessary gas mixture and temperature must be strictly selected.

Spectral interference occurs most often for two reasons.

First, there may be a sufficient proximity of the emission lines of different atoms of the analyzed sample, which, under the conditions of flame photometry, are perceived as radiation of the same type of atoms. For example, the most sensitive barium emission line (553.56 nm) coincides with the broad band emitted by CaOH. To solve this problem, high-resolution spectral dispersive systems should be used.

Secondly, spectral interference can also arise from the flame itself. Since the wavelength regions of such background radiation from the flames used are well known, interference of this type can be eliminated quite easily.

Ionization interference is a consequence of an undesirable process in the flame - the process of ionization of atoms of the substances under study:

Mg - ē → Mg +

The ionization process at high flame temperatures can go on and on until the complete loss of all electrons in the atom.

The resulting ions, like atoms, can be excited, and, accordingly, radiate the absorbed energy. However, of course, the characteristics of this radiation will differ from the radiation of excited atoms.

This circumstance complicates the analysis, since the course of the ionization process leads to a decrease in the concentration of the determined atoms, i.e. reduces the signal that needs to be tracked, and on the basis of which the calculation of the concentration is carried out.

This process is suppressed by introducing into the analyzed sample a salt of such a metal, the atom of which donates electrons more easily than the atom being determined.

Among the available salts that can be used for this purpose are cesium salts. They are able to generate an excess of electrons in the flame, and the ionization of more difficultly ionizable atoms to be determined is suppressed, i.e. the analyzed ions, with an excess of electrons, easily transform into atoms - into their analytic-active form.

It is possible to suppress the ionization of the atoms to be determined by lowering the temperature of the flame used. But as the temperature decreases, the concentration of excited atoms in the flame also decreases, which is undesirable.

Thus, the radiation of interest to us is caused by the transition of electrons from an excited state to the ground state, which is determined by the difference in the energies of electrons at different levels ∆E.

Naturally, for the vast majority of different atoms, ∆Е is also different.

Е 2 – Е 1 = hν = = , where

h is Planck's constant;

c is the speed of light.

Knowing ∆E (values ​​are tabulated), the radiation wavelength can be calculated.

If ∆E is expressed in eV, knowing ∆E, λ can be calculated.

For example, for calcium ∆Е = 2.95 eV, then

λ Ca = = 4200 Å

If the radiation of a flame containing calcium is passed through a monochromator and then photographed, then this image will have the following form and is called the emission spectrum:

Rice. 1. Line emission spectrum

Naturally, the more possible electronic transitions, the greater the number ν.

Such an image is called a line emission spectrum, which is the "spectral imprint" of an atom, because by the set of these lines, by their energies, one can determine which atom is present in the analyzed solution. Therefore, the spectrum is a powerful qualitative characteristic of a substance.

There are some dependencies: the higher the temperature, the more lines with higher transition energies are observed. At low temperatures, the most intense line will be determined by the transition of electrons from the first excited state to the ground state, for example, 3p → 3s.

This spectrum is suitable not only for qualitative, but also for semi-quantitative analysis with an accuracy of ± 0.5 orders of magnitude.

Semi-quantitative analysis is based on the fact that the disappearance or appearance of certain lines in the spectrum depends on the concentration of the substance. At the lowest concentrations only the thickest lines appear, at higher concentrations there are more lines, and at the highest concentrations much more. There are tables that give data on the concentration limits of the appearance or disappearance of certain lines, and this can be used for a semi-quantitative assessment of the concentration of a substance.

For the analysis of transition metals, a higher temperature flame is needed, since their excitation occurs only at high temperatures, which is ensured by the use of combustible mixtures consisting of nitrous oxide and acetylene, or oxygen and hydrogen.

4. Quantitative atomic emission analysis

Quantitative atomic emission analysis is based on the use of two types of instruments:

Atomic emission photometers

Atomic emission spectrophotometers.

With the help of these devices, either a sufficiently wide section in the spectrum is selected, containing not only the line to be determined, or a narrower section of the spectrum, containing only one line to be determined, and sent further to a photocell or LED.

The simplest scheme of an atomic emission photometer (often called a flame photometer) is as follows:

Rice. 2. Schematic diagram of a flame photometer

1 - containers with combustible mixture components, 2 - pressure regulators,

3 - spray chamber, 4 - burner, 5 - test solution,

6 - device for drying the spray chamber,

7 - focusing lens, 8 - entrance slit,

9 - a prism that separates the radiation along the wavelength, or a light filter,

10 - exit slot, 11 - photoelectric detector,

12 - recording device

There are certain requirements for a screen with a slit: the screen must be as wide as possible, and the gap as narrow as possible in order to pass without change only the radiation from the central part of the burner flame, i.e., so that the radiation is linear or close to linear.

Taking into account the fact that Li, Cs are scarce in nature, and K, Na are mainly found, especially since the difference in the emission wavelengths for K and Na is about 150 nm, the device is usually equipped with four light filters that pass that part of the spectrum, in which the radiation of only one of the given atom is located: a light filter for K, for Na, for Li, for Cs. A more complex system is the atomic emission spectrophotometer. An atomic emission spectrophotometer has one significant difference from a flame photometer: it contains a monochromatic system - a trihedral prism with a movable screen. The monochromatic system in an atomic emission spectrophotometer performs the same function as a light filter in an atomic emission photometer: it selects a certain part of the spectrum, which is then fed through a slit to a photocell. The fundamental difference between these devices is that the monochromator allows you to select a much narrower part of the spectrum than a light filter: a part with a level width of 2-5 nm, depending on the system used. There are systems that allow you to select an even narrower part of the spectrum - this is a diffraction grating. If you make it very large, then you can select a section of the spectrum with a width of 0.01-0.001 nm. Thanks to these capabilities, the atomic emission spectrophotometer makes it possible to study high-temperature flames in which there are many lines of various atoms. A multichannel atomic emission spectrophotometer has even greater analytical capabilities. Its schematic diagram differs in that after the monochromator in a multichannel atomic emission spectrophotometer there is not a photocell, but a diode bar, where up to 1000 diodes are placed in different positions. Each of the diodes is connected to a computer that processes the total signal and transmits an analytical signal (the current strength from each diode is measured).

Rice. Fig. 3. Schematic diagram of a multichannel atomic emission spectrophotometer: 1 – burner, 2 – entrance slit, 3 – prism, 4 – diode bar, 5 – recorder

The choice of information system may be different. In arc and spark variants of atomic emission spectrophotometry, the spectrum is recorded using a photographic plate, i.e., the spectrum itself is photographed. Spectrum analysis provides semi-quantitative information about the composition of a substance. Semi-quantitative analysis of a substance by the spectrum on a plate is based on the fact that the intensity of a particular line is logarithmically related to the concentration of the substance.

Quantitative methods are based on the summation of an analytical signal - an amplified photocurrent received from an LED or from a photocell, which is processed by a computer, or, in the simplest case, is fed to the dial of the device.

The strength of the photocurrent is related to the concentration through a proportionality factor:

The coefficient k will be constant at constant electrical characteristics of the system, as well as at constant concentrations of the analytic-active form in the flame.

The concentration of the analytic-active form in the flame depends on many parameters:

From the rate of aerosol supply to the flame, which, in turn, is determined by the gas pressure in the suction system of the device,

From the temperature of the flame, i.e. from the ratio of combustible gas - oxidizing gas.

However, in a narrow period of time, for example, within an hour, the coefficient k can be kept constant.

5. Spectrographic analysis

After obtaining the spectrum, the next operation is its analytical evaluation, which can be carried out by an objective or subjective method. Objective methods can be divided into indirect and direct. The first group covers spectrographic, and the second - spectrometric methods. In the spectrographic method, the photographic emulsion makes it possible to obtain an intermediate characteristic of the line intensity, while the spectrometric method is based on direct measurement of the spectral line intensity using a photoelectric light detector. In the subjective evaluation method, the sensitive element is the human eye.

The spectrographic method consists in photographing the spectrum on suitable plates or film using an appropriate spectrograph. The resulting spectrograms can be used for qualitative, semi-quantitative and quantitative analyses.

Spectrographic methods of spectral analysis are of particular importance. This is mainly due to the high sensitivity of the photographic emulsion and its ability to integrate light intensity, as well as the huge amount of information contained in the spectrum, and the ability to store this information for a long time. The instruments and equipment required are relatively inexpensive, the cost of materials is low, and the method is simple and easy to standardize. Spectrographic analysis is suitable for routine analysis and scientific research. Its disadvantage lies in the fact that, due to the complexity of photographic operations, it is not suitable for rapid analyzes, and its accuracy is lower than, for example, the accuracy of spectrometric or classical chemical analysis. Spectrographic analysis has been greatly developed, especially in the field of processing the huge amount of useful information contained in the spectrum with the help of an automatic microphotometer connected to a computer.

6. Spectrometric analysis

The spectrometric analytical method differs from the spectrographic method essentially only in the way the spectrum is measured. Whereas in spectrographic analysis the intensity of the spectrum is measured through an intermediate photographic step, spectrometric analysis is based on direct photometry of the intensity of the spectral lines. Direct measurement of the intensity has two practical advantages: due to the absence of a long processing operation of the photographed spectra and associated error sources, both the speed of analysis and the reproducibility of its results increase significantly. In spectrometric analysis, the operations of sampling, preparation, and excitation of spectra are identical to the corresponding operations of the spectrographic method. The same applies to all processes occurring during arousal, and spontaneous or artificially created effects. Therefore, they will not be discussed further here. The optical setup used in the spectrometric method, including the radiation source, its display, the entire dispersive system and spectrum acquisition, is almost identical to the spectrographic setup. However, a significant difference, which deserves separate discussion, lies in the method of supplying the light energy of the spectral lines to the photoelectric layer of the photomultiplier. The final operation of analysis, namely measurement, is completely different from the corresponding operation of the spectrographic method. Therefore, this stage of the analysis needs a detailed discussion.

7. Visual analysis

The third group of emission spectral analysis methods includes visual methods, which differ from the spectrographic and spectrometric methods in the way the spectrum is estimated and, except in rare cases, in the region of the spectrum used. The spectrum estimation method is subjective as opposed to the objective methods of the other two methods. In visual spectroscopy, the human eye is the receiver of light and uses the visible region of the spectrum from about 4000 to 7600 Å.

In the visual methods of spectral analysis, the preliminary preparation of samples and the excitation of their spectra essentially do not differ from similar operations of other methods of spectral analysis. At the same time, the decomposition of light into a spectrum is carried out exclusively with the help of a spectroscope. Finally, due to the subjectivity of the evaluation method, visual techniques differ significantly from spectrographic and especially spectrometric techniques. This also means that of the three methods of spectral analysis, the visual one has the least accuracy.

The detection limit of the visual method is relatively large. The most sensitive lines of elements, with the exception of alkali and alkaline earth, are in the ultraviolet region of the spectrum. Only relatively weak lines of the most important heavy metals are located in the visible region. Therefore, their limit of detection by the visual method is usually ten to one hundred times worse. Except in very rare cases, the visual method is not suitable for determining non-metallic elements, since their lines in the visible region are especially weak. In addition, the excitation of non-metallic elements requires special complex equipment and the intensity of the light source is not sufficient to evaluate the spectral lines with the naked eye.

In contrast to the disadvantages noted above, the great advantage of the visual method lies in its simplicity, speed and low cost. Working with the spectroscope is very easy. Although some training is required to evaluate the spectrum, simple analyzes can be learned quickly. The spectra can be assessed with the naked eye without the difficulties associated with indirect methods. This method is express: it usually takes no more than a minute to determine one component. The cost of relatively simple visual aids is low, and the cost of sample processing tool, counter electrode materials, and power is negligible. The techniques are so simple that after some training, analyzes can be performed by unskilled laboratory assistants. Due to the high rapidity of the method, labor costs per analysis are low. The economic efficiency of the method also increases due to the fact that the analysis can be carried out without destroying the analyzed sample and at the place where it is located. This means that with portable instruments it is possible to analyze intermediate products (eg metal rods), finished products (eg machine parts) or already installed products (eg overheated tubes of steam boilers) without sampling on site. Tools and time are also saved, organizational work is simplified and there is no need for destructive sampling methods.

The most important field of application of the visual method of spectral analysis is the control of metal alloys and mainly alloyed steels during their production for the purpose of sorting. The method is also used for the classification of metals or alloyed steels in the selection of valuable materials from scrap metal.

Conclusion

AES is a method for determining the elemental composition of a substance by optical line emission spectra of atoms and ions of the analyzed sample, excited in light sources. As light sources for atomic emission analysis, a torch flame or various types of plasma are used, including electric spark or arc plasma, laser spark plasma, inductively coupled plasma, glow discharge, etc.

AES is the most common express highly sensitive method for identifying and quantifying impurity elements in gaseous, liquid and solid substances, including high-purity ones. It is widely used in various fields of science and technology for the control of industrial production, exploration and processing of minerals, biological, medical and environmental research, etc. An important advantage of AES in comparison with other optical spectral, as well as many chemical and physicochemical methods of analysis, is the possibility of non-contact, rapid, simultaneous quantitative determination of a large number of elements in a wide concentration range with acceptable accuracy using a small mass of the sample.

Coursework: Physical and chemical bases of adsorption purification of water from organic substances

Non-state non-profit educational institution of secondary vocational education "Pokrovsky Mining College"

Test

Atomic emission spectral analysis

Completed:

group student

"Lab Analyst"

Profession: OK16-94

Chemical analysis laboratory assistant

Introduction

2. Atomizers

3 Flame processes

4. Quantitative atomic emission analysis

5. Spectrographic analysis

6. Spectrometric analysis

7. Visual analysis

Conclusion

Bibliography

Introduction

The purpose of practical emission spectrum analysis is to detect qualitatively, semi-quantitatively or accurately quantify elements in the analyte.

Spectral analysis methods, as a rule, are simple, rapid, easy to mechanize and automate, i.e., they are suitable for routine mass analyses. When using special methods, the limits of detection of individual elements, including some non-metals, are extremely low, which makes these methods suitable for determining trace amounts of impurities. These methods, except when only a small amount of sample is available, are practically non-destructive, since only small amounts of sample material are required for analysis.

The accuracy of spectral analysis, in general, satisfies practical requirements in most cases of determining impurities and components, with the exception of determining high concentrations of the main components of alloys. The cost of spectral analysis is low, although the initial investment is quite high. However, the latter quickly pay off due to the high productivity of the method and low requirements for materials and maintenance personnel.

Goals of the work:

1. familiarization with the theory of atomic emission spectral analysis;

2. learn to understand the main characteristics of the APP equipment;

3. study of AESA methods;

1. Atomic emission spectral analysis (AESA)

Methods of analysis based on the measurement of any radiation by the substance being determined are called emission methods. This group of methods is based on measuring the wavelength of radiation and its intensity.

The method of atomic emission spectroscopy is based on the thermal excitation of free atoms or monatomic ions and registration of the optical emission spectrum of the excited atoms.

To obtain the emission spectra of the elements contained in the sample, the solution to be analyzed is brought into the flame. The flame radiation enters the monochromator, where it is decomposed into individual spectral lines. With a simplified application of the method, a certain line is highlighted with a light filter. The intensity of the selected lines, which are characteristic for the element being determined, is recorded using a photocell or photomultiplier connected to the measuring device. Qualitative analysis is carried out by the position of the lines in the spectrum, and the intensity of the spectral line characterizes the amount of substance.

The radiation intensity is directly proportional to the number of excited particles N*. Since the excitation of atoms is of a thermal nature, the excited and unexcited atoms are in thermodynamic equilibrium with each other, the position of which is described by the Boltzmann distribution law (1):

(1)

where N 0 is the number of unexcited atoms;

g* and g 0 are the statistical weights of the excited and unexcited states; E - excitation energy;

k - Boltzmann's constant;

T is the absolute temperature.

Thus, at a constant temperature, the number of excited particles is directly proportional to the number of unexcited particles, i.e. in fact, the total number of these atoms N in the atomizer (since under real conditions of atomic emission analysis, the fraction of excited particles is very small: N*<< N 0). Последнее, в свою очередь, при заданных условиях атомизации, определяемых конструкцией и режимом работы прибора и рядом других факторов), пропорционально концентрации определяемого элемента в пробе С. Поэтому между интенсивностью испускания и концентрацией определяемого элемента существует прямо пропорциональная зависимость:

(2)

Thus, the intensity of the emission spectral line can be used as an analytical signal to determine the concentration of an element. The coefficient a in equation (2) is a purely empirical value, depending on the process conditions. Therefore, in NPP, the correct choice of atomization conditions and measurement of the analytical signal, including calibration according to reference samples, is of decisive importance.

The method is widely used for analytical purposes in medical, biological, geological, agricultural laboratories.

emission spectral atomization photometer

2. Atomizers

The main types of atomization and excitation sources are given in Table 1.

Table 1

The most important characteristic of any atomizer is its temperature. The physicochemical state of the analyzed substance and, consequently, the magnitude of the analytical signal and the metrological characteristics of the technique depend on the temperature.

Flame. The flame version of the method is based on the fact that the analyte in the form of an aerosol, together with the solvent used, enters the flame of a gas burner. In a flame with the analyzed substance, a number of reactions occur and radiation appears, which is characteristic only for the substance under study and is in this case an analytical signal.

The diagrams of the burners used in the flame photometry method are shown in fig. 1. The input of the analyzed liquid into the flame is usually carried out by its pneumatic spraying. There are mainly two types of atomizers used: angular and concentric, which operate due to the vacuum created above the opening of the atomizing capillary (or around it), the second end of which is immersed in the solution of the analyzed sample. The liquid flowing out of the capillary is sprayed with a jet of gas, forming an aerosol. The quality of the sprayer is evaluated by the ratio of the amount of liquid and gas (M W / M G) consumed per unit time.

Rice. 1. Torches for atomic emission flame spectrometry:

a) and b) a conventional Mecker burner and an improved burner: 1 - burner body; 2 - the surface on which the flame is formed; 3 - openings for the exit of combustible gases; 4 - supply of a mixture of combustible gases and aerosol; 5 - protrusion on the burner body with holes; c) combined burner with separation of the zones of evaporation - atomization and excitation of spectra: 1 - main burner with a ledge and holes in it; 3 - second additional burner with the same type or higher temperature flame; 4 - flame; 5 - zone of registration of radiation; 6 - supply of a mixture of combustible gases to an additional burner; 7 - supply of a mixture of combustible gases and aerosol to the main burner.

For the formation of a flame, a gas mixture is prepared, consisting of a combustible gas and an oxidizing gas. The choice of components of one or another gas mixture is determined, first of all, by the required flame temperature.

Table 2 contains information about the temperatures of various tribes in atomic emission analysis and their main characteristics.

Table 2 Characteristics of tribes used in atomic emission analysis

There are certain analytical characteristics of the flame. The flame, of course, must be stable, safe, and the cost of components to maintain it must be low; it must have a relatively high temperature and a slow propagation velocity, which increases the efficiency of desolvation and vapor production, and results in large emission, absorption, or fluorescence signals. In addition, the flame must provide a reducing atmosphere. Many metals in a flame tend to form stable oxides. These oxides are refractory and difficult to dissociate at normal temperatures in a flame. To increase the degree of formation of free atoms, they must be restored. Reduction can be achieved in almost any flame if the combustible gas flow rate is set to a greater than required combustion stoichiometry. Such a flame is called enriched. The rich flames produced by hydrocarbon fuels such as acetylene provide an excellent reducing atmosphere due to the large amount of carbon-containing radical particles.

The flame is the lowest temperature source of atomization and excitation used in nuclear power plants. The temperatures reached in the flame are optimal for determining only the most easily atomized and excitable elements - alkali and alkaline earth metals. For them, the method of flame photometry is one of the most sensitive - up to 10 -7% of the mass. For most other elements, the limits of definition are several orders of magnitude higher. An important advantage of the flame as a source of atomization is its high stability and the associated good reproducibility of the measurement results (S r - 0.01-0.05).

Atomic spectroscopy methods make it possible to determine the elemental composition of a test sample (the set of atoms present) from the absorption or emission spectra of light by excited atoms in the optical and X-ray ranges. Atomic spectra are observed in the form of bright colored lines and arise as a result of electron transitions from one energy level to others (Fig. 2.1); the number of levels in individual atoms is small, and therefore these spectra are discrete, that is, they consist of narrow individual lines. The simplest atomic spectrum is observed at the hydrogen atom, it has a set of lines called series: the Lyman series in the UV range, the Balmer series in the visible range, the Paschen, Bracket, Pfund and Humphrey series in the IR range. The frequencies of the lines in the hydrogen spectrum can be calculated from the differences in the energies of the corresponding energy levels. Other elements may have more spectral lines, but they are also narrow; each element is characterized by its own set of lines.

If the analyzed sample contains a number of elements, the frequencies of all lines can be measured and compared using a computer with the spectra of individual elements given in reference books. Thus, a qualitative analysis is carried out, and a quantitative one is based on measuring the intensity of the lines, which is proportional to the amount of the element in the sample.

Since the energy levels of valence electrons of free atoms and atoms that make up molecules differ markedly, to obtain atomic spectra, preliminary atomization (destruction) of the sample is necessary, that is, its transfer to a gaseous atomic state.

2.2.1. Atomic emission spectral analysis

A sample of the test substance is heated by plasma, electric arc or discharge, as a result of which the molecules dissociate into atoms, which partially pass into an excited state, the lifetime of which is about 10 -7 -10 -8 s, then spontaneously return to the normal state, emitting light quanta, giving a discrete spectrum of emission (emission). Measurement of the frequencies of the emitted lines in the emission spectrum and comparison with the spectra of individual elements of reference books allows you to determine which elements are contained in the sample under study. Quantitative analysis is based on measuring the intensities of individual lines of the spectrum, since the intensity of the radiation increases with increasing concentration of the element. Pre-calibration required. The method is very sensitive.

The main parts of an atomic spectrograph are shown in the block diagram

The excitation source can be an electric spark, arc, argon plasma or flame. The temperature of the electric arc is 3000-7000 ° C, sparks - 6000-12000 ° C, plasma - 6000-10000 ° C. The flame temperature is lower - from 1500 to 3000 ° C, therefore, not all compounds, but only some elements (alkaline , and etc.). A dispersive element that decomposes radiation into a spectrum - a prism or a diffraction grating. A photographic plate or photocell is used as a receptor.

More than 80 elements can be determined by this method; sensitivity varies from 0.01% (Hg, U) to 10 -5% (Na, B, Bi).